1. Field of the Invention
The present invention relates to a automatic automotive transmission and more specifically to a hydraulic spool valve control arrangement for such a transmission which has an accumulator which can be selectively connected with two different circuits by a selector valve.
2. Description of the Prior Art
JP-A-62-62047 discloses a transmission of the type which includes: a forward clutch, a high clutch, a band brake and a reverse clutch. The level of the line pressure which is used to engage these elements is controlled in accordance with engine load (as indicted by the throttle pressure produced by a throttle valve).
With this type of transmission, when it is conditioned for D-RANGE operation, 1st speed is produced by engagement of the forward clutch, 2nd speed is produced by engagement of the forward clutch and the band brake, 3rd speed is produced by engagement of the forward clutch and a high clutch, while 4th speed is produced by engagement of the high clutch and the band brake. When the transmission is conditioned to produce R-RANGE (viz., reverse) the reverse clutch alone is engaged.
As is conventional, shift valves are arranged to be responsive to the throttle pressure (engine load) and a governor pressure which is representative of vehicle speed, and to control the above listed engagements in a manner to selectively produce 1st, 2nd, 3rd and 4th forward gears.
It should be noted that in this type of transmission the band brake is operated by a servo having a 2nd speed apply chamber, a 3rd speed release chamber and a 4th speed apply chamber. With this particular arrangement when the 2nd speed apply chamber is supplied with line pressure, the band brake is applied, when the 3rd speed release chamber is pressurized, the band brake is released irrespective of the presence of line pressure in the second speed apply chamber. When the 4th speed apply chamber is pressurized the brand brake is applied irrespective of the presence of line pressure in the 3rd speed release chamber.
When any of the above mentioned friction elements are supplied with line pressure and engaged, a substantial shift shock tends to be produced. In order to overcome this problem the arrangements depicted in FIGS. 4 and 5 have been proposed.
In the FIG. 4 arrangement, when an upshift to third speed is made, the high clutch and the 3rd speed release chamber 3S/R are supplied with line pressure. In order to attenuate shift shock a 2-3 accumulator 2 is provided.
In the FIG. 5 arrangement when an upshift to 4th speed is made, line pressure is supplied to the 4th speed apply chamber 4S/A. In order to attenuate shift shock which tends to be produced, a 3-4 accumulator 4 is provided.
The accumulators 2 and 4 are supplied with control pressures which respectively comprise line pressure and a pressure which is modified in accordance with the load on the engine. The latter mentioned pressure is produced by a non-illustrated accumulator control valve which is responsive to throttle pressure.
The 3-4 accumulator is provided with a spring 5 which urges the stepped accumulator piston in a direction which tends to minimize the volume of a chamber which is fluidly communicated with the reverse clutch R/C. When a N-R selection is made, the pressure which is supplied to the reverse clutch R/C is temporarily reduced by the filling of the chamber and the resultant stroking of the accumulator piston. Accordingly, this arrangement is also capable of reducing N-R select shock in addition to 3-4 shift shock.
However, with the above mentioned 3-4 accumulator the accumulator control pressure has a large effect on the stroking characteristics of the accumulator piston during a N-R select. When the engine speed is low the control pressure is low, and it is possible to select the spring 5 in a manner wherein the appropriate resistance to the stroking of the accumulator piston is possible. However, when the engine idling speed is high, the accumulator control pressure increases. As the accumulator control pressure acts in a direction opposite to the bias of the spring 5, the appropriate resistance to the stroking of the accumulator piston is lost resulting in N-R select shock attenuation being reduced.